Emulsion and solution polymers find wide application as adhesives, binders and coatings. Unfortunately, many of these polymeric materials, especially those prepared predominantly from vinyl acetate, ethylene, vinyl chloride, or their mixtures, show inadequate resistance to water and other solvents in their everyday use. In particular, they experience substantial and unacceptable loss in strength in the presence of solvents such as perchloroethylene, methyl ethyl ketone and toluene. In addition, many of these polymers exhibit deficiencies in adhesion to the substrates on which they are used, for example vinyl acetate, ethylene or vinyl chloride polymers on glass, metal or polyester. These deficiencies are reduced, especially for relatively hydrophilic monomers, by the use of adhesion promoting or crosslinking comonomers and/or post-added crosslinkers.
By far the most successful crosslinking materials are aminoplasts, especially N-methylolacrylamide and urea-formaldehyde condensates. These materials have met substantial success because they are low in cost, highly compatible with aqueous emulsion systems, rapidly cured under acid catalysis, and substrate reactive in that, for example, they react with the hydroxyl groups of cellulosic materials. These crosslinking materials, however, suffer from two deficiencies: (1) the emission of low levels of formaldehyde during cure and subsequent use, and (2) inadequate adhesion to certain substrates, for example, metal, glass and polyester.
Many attempts have been made to overcome or minimize the first deficiency, especially after the potential carcinogenicity and irritant properties of formaldehyde became widely recognized.
To reduce the level of formaldehyde in emulsion products, the use of O-alkylated N-methylolacrylamides such as butoxymethylacrylamide or the use of about equimolar ratios of N-methylolacrylamide with acrylamide were introduced. These materials did not, however, totally eliminate the presence of formaldehyde.
Acrylamide/glyoxylic acid condensates and their ethers and esters have been used. These materials have not performed well in applications with vinyl acetate-ethylene emulsions, especially on nonwoven cellulosic substrates. The use of amide/glutaraldehyde condensates, for example the condensate with acrylamide, has also been attempted. The combination of the reagents, however, gave a complex mixture of uncharacterizable products which did not perform well in textile crosslinking applications.
Epoxide functional comonomers such as allyl glycidyl ether, glycidyl (meth)acrylate or their precursors have also been used. These compounds suffered from high costs, limited shelf stability of the functionalized emulsion polymer and toxicity questions associated with epoxide materials.
Other approaches have used esterification chemistry (carboxylic acid plus alcohol to give an ester crosslink), but such approaches require a slow and expensive high temperature curing cycle. Post-addition of formaldehyde-free urea/glyoxal condensates including N,N'-dialkyl-4,5-dihydroxyimidazoles has been used in Japan for fabric treating but such systems are less efficient than formaldehyde-containing analogs.
Thus there is a need for functional monomers which, after incorporation into polymers or copolymers, can be crosslinked under mild conditions with themselves and/or other polymer or substrate reactive groups to give binders, adhesives and/or coatings with high water and solvent resistance and good substrate adhesion. Such products should also be formaldehyde-free.